Histamine release from mast cells and blood basophils of several different species is studied as one of the immunological mechanisms involved in inflammation. The generation of histamine releasing substance in human serum by microbial cell walls or by antigen-antibody complexes has been studied. This mechanism of basophil activation has been compared with that of IgE-mediated release and found to be different in several ways. The role of serum factors including complement in the liberation of histamine from human basophils is being studied.